FCGRT and acquired polycythemia vera: However, since mouse IgG exhibit only negligible binding to human neonatal Fc receptor (FcRn) [12,13], and 4B3 has so far only been used as a single-chain fragment, we here aimed to develop antibody tools that are able to interact with human FcRn and would allow the role of FcRn in the pathogenesis of PV in human keratinocytes to be studied.